1. Field of the Invention
The present invention is generally related to a transition metal complex, especially to a transition metal complex having carbene ligands, and applications thereof in organic electronic devices.
2. Description of the Prior Art
Organic light-emitting diode (OLED) is an electronic device which emits light through the use of organic semiconductor materials and emitting materials. OLED works on the principal of electroluminescence, where a bias is applied to an electrode pair causing the electrons and holes to diffuse through an electron transport layer (ETL) and hole transport layer (HTL), respectively, to enter an emitting material region. The electrons and holes recombine in the emitting region and form a particle generally referred as exciton. In order for the exciton to come back to the ground state, the energy is given off in the form of photo radiation. The color of the radiation can be tuned by using different emitting materials.
Recently, organic emitting materials with an emissive triplet state, also referred to as organic phosphorescent materials, have drawn much attention as an OLED material. Theoretically, phosphorescent materials are three times in efficiency compared to conventional fluorescent materials which have an emissive singlet state. Common organic compounds have either no phosphorescent properties or can only be observed of this property at a very low temperature, such as 77 K. Investigation has suggested that addition of heavy atoms can overcome this drawback; however, not all heavy atoms are suitable for this purpose. Researchers have reported that transition metals such as rhenium (Re), osmium (Os), iridium (Ir) and platinum (Pt) are the most suitable candidates.
Metal complexes have been used as the phosphorescent dopants of OLEDs. In some metal complexes, the presence of heavy atoms causes strong spin-orbital coupling, leading to the mixing of the singlet and triplet excited states. This greatly reduces the lifetime of the triplet state and thereby the phosphorescence efficiency is promoted. Among these metal complexes used in the light-emitting layer of the organic light emitting diode, iridium complexes have been extensively researched due to the strong spin-orbit coupling resulting from their electron configurations.
Cyclometalated iridium complexes are the one of the most efficient and bright organic phosphorescent materials currently known. Among them, Iridium(III) bis(4,6-difluorophenylpyridinato)picolate (FIrpic) is a common blue phosphorescent material having triplet state. FIrpic renders high device efficiency, but its color saturation is poor. Its CIE coordination locates in the region around (0.16, 0.30). The high Y value leads to a color that lies between blue and green, rather than a deep blue color which is required in a full-color display.
In 2005, professor Thompson announced a novel iridium metal complex which has carbene ligands1,2. The disclosed material gave a CIE coordination of (0.17, 0.06), not quite met the required (0.15, 0.15) or (0.15, 0.09); moreover, the external quantum efficiency only reached 5.8%, and its brightness only reached 1.7 μm/W. More similar compounds were disclosed in 2005, 2006 and 2007, with substantially same structure, only differed in the substituents on the carbene ligands.
It has been no longer than 7˜8 years since the first phosphorescent material was reported, in which the development of blue phosphorescent material has only been of 3˜4 years. Recently, phosphorescent material has been a key to technical breakthrough for OLED devices. Especially, the OLED-based white lighting really depends on availability of more efficient and saturated blue phosphorescent materials.
In summary, blue phosphorescent material plays an important role in whether OLED can applied in the next generation white lighting. Therefore, a blue phosphorescent material having high external quantum efficiency, great hue, brightness and saturation is mostly desired by the industry.